Electric discharge lamp



G. F. BRETT ELECTRIC DISCHARGE LAMP Filed Feb. 19,

lNvI-:NTQR` G, F- XBRETT 72 "gigi/v m ATTORNEY Patented Apr. 30, 1935PATENT oI-ElcE ELEc'rmo DISCHARGE LAMP j v George Fairburn Brett, Leeds,England, assignor to Radio Corporation of America, a corporation ofDelaware Application February 19, 1932, Serial No. 593,979y

In Great Britain February 2l? 1931 2 Claims.

- electrodes, i. e. electric discharge lamps comprising an anode,cathode and a further electrode.

Two of the principal disadvantages in ordinary two electrode glowdischarge lamps are that a comparatively high striking voltage isnecessary while the current vvoltage characteristic curves` exhibit aconsiderable hysteresis effect, i. e. the curve obtained for ascendingvalues of voltage is not 'the same as that obtained for descendingvalues. This hysteresis eect is usually associated with instability at:low current values atV which values the vluminous discharge may suddenlycease altogether without apparent tcause- Further-'where a lamp isemployed' for picture recording or television or like purposes, badreproduction generally results, since the current l at the strikingvoltage may be different on two different occasions. 'l'hese eects ofinstability and lack oi reproducibility become especially noticeablewhen the dischargepath is long and constricted, and therefore theseeffects are particularly serious in discharge lamps suitable for use inpicture recording andte'levision, since as is well known the dischargepath in such lamps is almost always long and constricted.

According to this invention an electric discharge lamp comprising ananode and a cold cathode is provided with an auxiliary electrode whichis of very small area relative to the cathode proper.

The invention is illustrated in several of its preferred forms by theaccompanying drawing, wherein:

Fig. l represents a vertical section through a discharge tube inaccordance with a preferred modification of my invention, and whereinFigs. 2 through 5 inclusive represent sectional views through theelectrode structure of modined forms of the invention set forth by Fig.l. By these figures, Fig. 2 represents the use of an auxiliary electrodeformed as a loop or ring of wire.; 3 represents a similar formation ofauxiliary and anode electrodes, each formed from a ring or loop of wire;Fig. l represents the use of an auxiliary electrodeof cylindricalformation which surrounds the space between the cathode and anode; andFig. 5 is a modification of the showing of Fig. 2 wherein the anodemember' is anode of a discharge lamp in accordance with arrangedopposite the opening in a cylindrical cathode. v

Referring to Figure 1 anA electric' discharge lamp comprises a cathodeCof fairly large area made in the form of an open ended cylinder ofnickel iron or other suitable metal. Co-axial with lthe cathode andclosely adjacent thereto is an auxiliary electrode AE consisting of acylinder of the same diameter as the cathode, but of very much shorterlength, the area of the auxiliary electrode being thus much smaller thanthat of the cathode. Closely adjacent the auxiliary electrode is acylindrical anode A which is again coaxial with the lother twoelectrodes and is again constituted by-a cylinder of the same diameter'.M The anode cylinder need not be long and may conveniently beintermediate in length between the length: of the cathodev and thelength of the auxiliary. electrode. All three electrodes may consist ofnickel or other suitable metal and are mounted in a glass bulbcontaining a suitable gas mixture at a pressure of a. few millimetri-esof mercury. a y

In a modification shown in Figure 2,4the auxiliary electrode AE may beconstituted by a ring of wire instead of by .a short cylinder; or, asshown in Figure 3 the auxiliary electrode AE and the anode A may each beconstituted by a ring of wire.

In a further modification, the electrodes are so arranged that the lightfrom the discharge in the auxiliary electrode circuit seen in thedirection of the cathode is very small. Such a modication is shown inFigure 4 in which the auxiliary electrode AE is inthe form of a collar1r the small gap between the anode A and the cathode C.

The cathode may be mounted in a glass shield or sleeve S (see Figure l)and the leads to the cathode and the auxiliary electrode and if desired,to the anode also, should be glass covered in order to prevent dischargetaking place, otherwise than in the desired manner. The glass coveringis not shown in the drawing.

In the rnodicationshown in Figure 5 the anode 45 A is in the form of aplate or disc opposite the end of the cathode C and on the other'sidethereof from the auxiliary electrode AE. if desired... the positions ofthe auxiliary electrode andthe this invention may be interchanged. Suchinterchanging has the eect of lowering the striking voltage but thestability may be found to be somewhat reduced ascompared to thatobtaining in the arrangement illustrated.

that it is very easy to construct and requires" very little metal.

It will be noted that the glass sleeveS shown in Figure 1 is extended alittle way beyond the end of the cathode C. This extension serves toreduce blackening of the envelope opposite the cathode due to cathodesputtering and therefore has the effect of increasing the useful life`of the lamp.

The arrow in Figure 1 indicates the direction from which the lamptherein shown is intended to be viewed.

When electrical discharge lamps in accordance with this invention are inuse a continuous small localized discharge is maintained inthe lamp fromthe auxiliary electrode; this discharge acting as a trigger for the maindischarge between f the cathode and anode and causing the main dischargeto occur and be extinguished smoothly for all values of operatingcurrent.

It will thusbe seen that a small change in the current between theauxiliary electrode and the anode has little or no effect in thedischarge be' tween the cathode andthe anode, while even a largerelative current change in the circuit of the lfor the three electrodesare separated from one another only by a very small space; in practiceseparations of one half millimetro between adjacent electrodes has beenfound satisfactory.

Again, owing to the relative size and arrangement of the electrodes thetotal auxiliary current is always very small and in one practical casehas only been about one half milliampere, i. e. about 2% of the maximummain current.

Having now described the invention, what is claimed and desired toprotect by Letters Patent is the following:

1. An electric discharge tube comprising a tubular anode electrode, atubular cathode electrode axially separated from the anode, and atubular auxiliary electrode positioned between the anode and cathode4and surrounding the space between the anode and cathode. and anenvelope enclosing all of the electrode elements.

2. An electric discharge tube comprising a tubular anode electrode, atubular cathode electrode axially separated from the anode, and aringlike auxiliary electrode positioned between the anode and cathodeand surrounding the space between the anode and cathode, and an envelopeenclosing all of the electrode elements.

. GEORGE FAIRBURN BRE'I'I.

